Generation of optical signals with RF components

ABSTRACT

An optical beam having a high radio-frequency modulation is generated by generating a lower frequency modulation, using it to control the optical output of a laser and further modulating the optical output in an optical modulator by a control signal having another lower frequency modulation generated. Either or both of the lower frequency modulations also carries an information containing modulation. the effect of the optical modulator is to up-convert the modulation carried by the optical beam by the modulation frequency of the control signal. The optical modulator may be a Mach-Zehnder interferometer. The non-linearity of such a modulator with respect to its control input may be exploited by selecting the amplitude of the control signal such that the optical output is up-converted by an integer multiple of the initial modulation frequency. These methods avoid the need to apply the high frequency modulation to either the laser input or the control input directly.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of generating signals havingoptical carriers. It finds particular application in the generation ofradio frequency (RF) modulations to be carried over optical media.

2. Related Art

The principle of modulating an optical beam by an information content iswell established, and various methods are known of achieving this. Somearrangements involve controlling the light source (typically a laser) byvarying its input bias voltage. Other arrangements use optical devicesin the path of the generated beam (typically on an optical fibre) tointerrupt the beam. A known optical device for this purpose is theMach-Zehnder interferometer. The principle of this device is to splitthe optical beam into two paths, one or both of which passes through amedium wh refractive index varies as a function of the electricpotential applied to it. By applying an electrical signal one or bothpaths, the difference between the path lengths of the two beam paths canbe varied, such that the re-combined beams interfere constructively ordestructively depending on the electrical fields applied. The intensityof the recombined beam therefore varies in response to the varyingelectrical input signal.

It is known for the modulation carried by an optical signal to include acarrier frequency in the radio frequency (RF) range. This principle,known as `Radio by Fibre,` allows a radio signal, including its RFcarrier, to be generated at one location and transmitted over the airfrom another, remote, location. The signal is typically carried by anoptical fibre between these locations. This allows the equipment at thepoint from which the signal is to be transmitted over the air to be keptvery simple. In its simplest form it need consist only of a detector toconvert the optical input into an electrical signal, and an antenna fortransmitting the electrical signal over the air. This is particularlyuseful in situations where an antenna has to be located at a pointdifficult of access, such as a hilltop, because the complex equipmentrequired to generate the radio frequency carrier in particular the localoscillator can be located at another more accessible location. Moreover,it is possible to achieve economies in a branched network, in which onesignal is transmitted to several antenna sites, because only one localoscillator is required to generate the carrier to be transmitted by allthe antennas.

The detectors for these arrangements are typically photodectors. Theseproduce an electrical output which varies with the intensity of incidentlight. This electrical output therefore corresponds to the modulation,but without the optical carrier frequency.

Known optical systems suffer from a number of practical limitations, inparticular in the accurate transmission of high radio frequencies (ofthe order of a few tens of GHz). As frequencies approach the millimeterwaveband (tens of gigahertz) it becomes increasingly difficult toachieve direct modulation of the laser by applying a signal to the inputbias voltage, because of inherent physical limitations of the laserdevices themselves. Similar constraints apply to modulation devices suchas the Mach-Zehnder interferometers discussed above, as the highfrequencies necessitate very small dimensions which impose constraintswhich reduce their efficiency. Velocity matching between the electricaland optical signals also becomes harder to achieve and maintain.

There is an additional problem with the application of signals by meansof a Mach-Zehnder optical modulator. As explained above, the principleof these modulators is that as the voltage applied to the electricalinput of a Mach-Zehnder interferometer is increased, the difference inoptical path length increases. This results in the two optical pathspassing in and out of phase, so that the amount of light passing throughthe interferometer varies as a periodic function of the applied voltage,and not as a linear function. This non-linear response to the inputmeans that only constant-amplitude signals would be accuratelyreproduced.

A proposal by O'Reilly and Lane (Electronics letters, Vol 28, No 25,page 2309) addresses the first of these problems. In this proposal, anelectrical signal having a frequency ω in the RF band is applied to thecontrol input of an optical modulator. This modulator is biassed suchthat the optical output generated is modulated by a signal dominated bytwo side bands centred on the optical carrier frequency of the originaloptical signal fed into the modulator and each spaced from the opticalcarrier frequency by the frequency of the electrical signal ω. Theseside bands produce sum and difference beats at an optical receiver suchas a photodector. The `sum` beat is at twice the optical carrierfrequency. The `difference` beat is at frequency 2 ω which is in the RFband. A photoelectric receiver would not be sensitive to theoptical-frequency `sum` beat, but would generate an electrical signal atthe `difference` frequency. This proposal therefore produces an outputsignal carried by the optical beam which is at twice the frequency ofthe electrical signal applied to the control input.

O'Reilly and Lane further propose applying an information-containingmodulation to this output signal by separating the two side bands usingoptical filters, modulating one of them with the information contentusing a second optical modulator, and re-combining them, to generate anoutput having an optical carrier, modulated by a second, radiofrequency, carrier 2 ω, and further modulated by the informationcontent. However, this further optical modulation suffers from thenon-linearity discussed above, and requires the use of optical filtersand a second optical modulator which result in optical losses.

SUMMARY OF THE INVENTION

According to a first aspect of the invention there is provided a methodof generating a modulated optical signal having a first RF componentincluding a first RF carrier frequency and an information component, themethod comprising the following steps:

i) Generating a first optical signal having a second RF componentincluding a second RF carrier frequency different from said first RFcarrier frequency;

ii) Generating a control signal having a third RF component including athird RF carrier frequency different from said first RF carrierfrequency;

said second or said third RF component including the informationcontent;

iii) applying the first optical signal to an optical modulator and

iv) applying the control signal to the optical modulator to modulate thefirst optical signal so as to produce an output optical signal modulatedby said first RF carrier frequency and said information component, saidfirst carrier frequency being said second carrier frequency upconvertedby said third carrier frequency or by an integer multiple of said thirdcarrier frequency.

The invention has a number of advantages over the prior art. By applyingRF components to both inputs of the modulator, an upconversion isachieved. This allows a higher frequency to be generated at the outputof the modulator than is applied to either of the inputs. The use oflower RF frequencies in the optical input allows a simpler laser to beused to generate the desired RF signal. The use of lower RF frequenciesat the control input of the modulator similarly allows greaterflexibility and simplicity in the design of the modulator, which isconstrained by the input frequency and not the output frequency.

Either the first optical signal or the control signal may include theinformation component. The invention also extends to a method ofencryption comprising the steps of the first aspect wherein both theinput optical signal and the control signal include an informationcomponent, one of which is a predetermined encryption code, and to amethod of de-encrypting a signal generated in this way, comprisingapplying to the output signal a further modulation complementary to thatof the predetermined encryption code. This provides a simple way ofupconverting and encrypting a signal in one operation.

In a preferred embodiment the control signal is an electrical signal,the modulator being of the tape where the optical output of themodulator has a non-linear response to the electrical control input. AMach-Zehnder interferometer is an example of such a modulator. Using amodulator of this type the first RF carrier frequency may simply be asum of the second and third RF carrier frequencies (i.e the third RFcarrier frequency upconverts the second RF carrier frequency to thefirst RF carrier frequency). However in this type modulator theamplitude of the control signal may be select such that the second RFcomponent is upconverted by frequency which is a desired integermultiple of the third component. This allows even larger conversionfactors to created between the control frequency and the outputfrequency, with consequent lower frequency control signals furthermitigating the design constraints on the modulator.

The modulation carried by the first optical signal is preferablygenerated by controlling the bias voltage of a laser.

The invention also extends to an optical signal generated according tothe method of the invention, and a radio or electrical signal generatedby detecting such an optical signal.

According to a second aspect the invention comprises apparatus forgenerating an optical signal having a first RF component which includesa first RF carrier frequency and an information component, comprising;

i) an optical modulator having an optical input, an optical output, anda control input;

ii) means for supplying to the optical input a modulated optical signalhaving a second RF component including a second RF carrier frequencydifferent from said first RF carrier frequency;

iii) means for supplying to the control input a control signal having athird RF component including a third RF carrier frequency different fromsaid first RF carrier frequency,

iv) means for applying a modulation comprising said informationcomponent to said optical signal or said control input;

the arrangement being such that there is produced at the optical outputan optical signal modulated by said first RF carrier frequency and saidinformation component, said first carrier frequency being said secondcarrier frequency upconverted by said third carrier frequency or by aninteger multiple of said third carrier frequency.

In a preferred embodiment the control input is an electrical input andthe optical modulator is a Mach-Zehnder interferometer. The means forgenerating the control signal may be arranged to generate a signal ofsufficient amplitude that the second RF component is upconverted by afrequency which is an integer multiple of the third RF component.

The means for supplying the optical signal is preferably a laser.Preferably means for controlling the bias voltage of the laser areprovided for generating the second RF component.

The input optical signal therefore carries an initial RF modulation, sothat the output optical signal comprises an RF modulation which is theinitial RF modulation upconverted by the control RF frequency. Thisallows the use of control frequencies lower than the desired output RFcarrier frequency. Moreover, if a modulator having a non-linear transferfunction is used, such as a Mach-Zehnder interferometer (whose transferfunction is periodic) even larger upconversion factors can be usedbecause by selection of the amplitude of the control signalappropriately, the output can be dominated by a harmonic of the controlsignal. To understand why this occurs, consider a control signal havinga amplitude V₂π causing the difference in path lengths in the opticalpaths to vary between zero and one wavelength λ. (See FIG. 2). On eachcycle of the control signal the path length difference will thus varyfrom zero to λ and back again. There will thus be two points ofconstructive interference (at zero and V₂π) and two points ofdestructive interference (at V.sub.π) for each cycle of the controlsignal, so that the signal applied by the modulator to the opticalsignal is in this case twice the control signal frequency. By selectingother amplitudes for the control signal different integer multiplicationfactors can be introduced. In the simple example above the amplitude ofthe control signal is chosen to vary the path length by a whole numberof wavelengths. Varying it over smaller amplitudes can also generatesignals having dominant harmonics which may be used in the same way.

The invention also extends to an encryption device comprising theelements of the second aspect of the invention wherein means areprovided for applying a modulation comprising an encryption code to theoptical input or the control input.

Information content may be applied to either input signal. In modulatorssuch as Mach-Zehnder interferometers having the non-linearity referredto above, large amplitude modulations of the control input would not beaccurately reproduced in the output optical signal. However amplitudemodulations applied to the optical input do have a linear response. TheMach-Zehnder interferometer used as the modulator of the preferredarrangement also has a linear response to phase and frequencymodulations applied to either the control or the optical input.Combinations, such as phase-amplitude modulation (e.g quadratureamplitude modulation: QAM) are also possible in the optical input.

According to a third aspect of the invention, there is provided a methodof generating an output optical signal having an RF modulationcomprising applying an input optical signal to an optical input of amodulator having a non-linear transfer function, applying a controlsignal having a control RF frequency to a control input of themodulator, the amplitude of the control signal being such that theoutput optical signal is modulated by an RF frequency which is aninteger multiple of the control RF frequency. The modulator ispreferably a Mach-Zehnder interferometer. This aspect of the inventionallows high RF frequency modulations to be applied to the optical signalalthough a lower frequency is applied to the control input. This has theadvantages discussed above in mitigating design constraints on theoptical modulator.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be further described by way of example only withreference to the accompanying drawings in which;

FIG. 1 is a diagrammatic representation of an arrangement for performingthe method of the invention;

FIG. 2 is a diagram showing the variation of transmissivity of aMach-Zehnder modulator to changes in applied voltage; and

FIGS. 3 to 6 are diagrams showing the way in which the frequencymultiplication factor changes with the amplitude of the voltage appliedto a Mach-Zehnder modulator.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Referring to FIG. 1, there is shown an arrangement including a laser 1having an electrical power input 2, and an optical output 3. A source 4aof an RF electrical signal is connected to the electrical input 2through amplifier 5. Connected to the optical output 3 of laser 1 is aMach-Zehnder optical modulator 6. The electrical input 7 of themodulator 6 is fed another RF signal from source 4b through a poweramplifier 9. The output 10 of the modulator 6 is connected, throughoptical fibre 11 to a photodetector 12 which converts the optical signalto an electrical signal. The photodetector is coupled through anotheramplifier 13 to a radio antenna 14 which converts the electrical signalto a radio signal. The RF electrical signals from either of source 4a orsource 4b may include a modulation carrying the required informationcontent. Signal source 4a may generate analogue or digital modulationoutputs which may themselves be modulated onto RF carrier frequencies.Signal source 4a may generate a multichannel output using any suitablemodulation scheme such as frequency modulation, amplitude modulation, orphase modulation. Because of the non-linearity of the modulator, source4b can only supply one channel at a time. This channel may be frequencyor phase modulated.

Several ways in which this arrangement may be used will now bedescribed.

In a first method the signal source 4a generates an electrical FMcarrier in the gigahertz range suited to the response time of thelaser 1. The FM electrical carrier signal carries a modulation in themegahertz band, provides the input to the laser 1, the optical signalgenerated by the laser 1 varying directly in response to the electricalinput signal to provide a modulated FM optical signal at output 3.

In order to up-convert the optical signal modulated an intermediatefrequency (IF) to a higher frequency optical signal is mixed inmodulator 6 with a loca oscillator frequency LO from source 4b. Theoutput of modulator 6 is thus a signal comprising an optical carrier,modulated by a high frequency RF signal being the RF carrier frequency(IF) from source 4a plus the local oscillator frequency of source 4b,itself modulated by the information content. Upconversion thereforetakes place within the optical system, and this has a number ofadvantages over performing it in the electrical systems upstream ordownstream of the optical system. In a second arrangement, theinformation bearing modulation may be applied through signal source 4b.This can be phase or frequency modulated and has an RF carrierfrequency.

By supplying the signal from source 4b at a sufficiently large amplitudethe RF frequency may be multiplied in the modulator 6 in a manner to bedescribed below, allowing the output of the modulator to be at a higherfrequency than the electrical input, thereby avoiding the problemsassociated with optical modulators when driven at such high electricalfrequencies.

Other arrangements can be devised which are within the scope of thisinvention. For example, information-bearing signals may be applied byboth sources 4a and 4b, that at source 4b being a predetermined code.The two signals will become scrambled in output 10. A remote user,knowing the code signal applied at 4b may receive the scrambled signalover the air from antenna 14, and re-combine the output signal by asignal complementary to that from source 4b, to recover the signal frominput 4a.

The use of the modulator as a multiplier of the local oscillatorfrequency, in addition to its function as a mixer of the localoscillator and optical modulation signals, will now be described.

In FIG. 2 the horizontal axis shows the voltage applied to the modulatorand the vertical axis shows the transmissivity of the modulator. It willbe seen that the response of the modulator is highly non-linear withrespect to variations in electrical input. It is therefore most suitedfor provision of a constant amplitude modulation. More complexmodulations will be distorted by the non-linear response. Inarrangements in which the modulated input 4 is fed to the electricalinput 7 of the optical modulator 6, this non-linear response limits useof the system to single channel applications.

In the arrangement according to the present invention, the non-linearresponse of the modulator can be used to generate harmonics of the localoscillator frequency, thus allowing higher upconversion factors in themodulator.

The output i(t) of the photodector 12 can be expressed as a sum of itsFourier components I_(p) : ##EQU1##

Assume a Mach-Zehnder modulator with a characteristic symmetrical aboutV=0 (FIG. 2), with V.sub.π being the voltage excursion required for thetransmissivity of the Mach-Zehnder interferometer to go from fulltransmission (constructive interference) to full extinction (destructiveinterference).

Applying a sinusoidal voltage (V_(a) sin ωt+V_(b)) to an interferometerwhere:

V_(a) =amplitude of applied voltage

V_(b) =d.c bias of applied voltage

gives an output having harmonics whose Fourier amplitudes are then givenby: ##EQU2##

By selecting the bias voltage to be 1/2(V.sub.π) we can generateeven-only harmonics. By selecting the bias voltage to be V₀ we canselect odd-only harmonics. Selecting V.sub.δ =0 reproduces theMach-Zehnder transfer function in the zeroth harmonic as the biasvoltage V_(b) is tuned, and zero for the higher harmonics.

The d.c amplitude is equal to |I₀ |. (ie the mean light powertransmitted through the modulator)

The a.c amplitude is equal to |2I_(p) |, p>1.

Thus we can define the modulation depth for the "p'th" harmonic as:##EQU3## However the value of I₀ changes with applied a.c. modulationvoltage V_(a) (as well as with bias voltage V_(b)). Therefore,maximising the modulation depth does not necessarily correspond tomaximising the amplitude of a particular harmonic.

It is perhaps more convenient to choose the d.c. level when V_(a) =V_(b)=0 as the reference, in which case I₀ (V_(a) =V_(b) =0)=1.

Then our modified modulation depth becomes: ##EQU4##

FIGS. 3 to 6 show transfer characteristics calculated for various valuesof V_(a) and V_(b). Odd harmonics are biassed at 1/2V.sub.π, even onesare biassed at V=0. In these figures the input voltage is shown as adotted line and the output as a solid line. The voltages applied(arbitrary units) are given in the Table below:

    ______________________________________                                        Occc cccc 1000                                                                         0000 10nn nnnn                                                        Character                                                                             Sequence tag                                                                              Number                                                   ______________________________________                                    

In this way, by applying different amplitudes V_(a) to electrical input,modulations of different frequencies can produced in the optical system.

As will be seen, the output wave form is not the same shape as theinput. It can therefore be seen from these Figures that multiple-channelsignals applied to such a modulator input would be distorted and thusdifficult to extract at the receiver. However, for a single-frequencyinput such as a local oscillator this is not important as unwantedharmonics can be filtered out downstream.

In the simple case described with reference to FIGS. 3 to 6 the opticalinput is unmodulated, so that the optical output is modulated only bythe multiplied control frequency. However, if the optical input signalalready carries a modulation, the optical modulator will mix thismodulation with the multiplied control frequency to provide anupconversion.

EXAMPLE

In the exemplary embodiment of FIG. 1, the signal source (4a) wasembodied by an Avantek VTO 9090 oscillator generating channels between950-1750 MHz. The output from this was used to modulate a Lasertron QLXS1300 MW laser (1), whose output was directed along a step-index singlemode 9/125 μm optical fibre (3) to a BT&D IOC 2000-1300 modulator (6).The control input (7) to this modulator was supplied by a Marconi 2042local oscillator (4b) working at 3.4 GHz and amplified by a MinicircuitsZFL/42 amplifier (9) such that the eighth harmonic of the localoscillator frequency (ie 27.2 GHz) dominated the signal response. Theoutput of modulator 6 thus had a RF carrier frequency of 27.2 GHz+(950to 1750 MHz) or 28.15 to 28.95 GHz, which was fed through anotherstep-index single mode 9/125 μm optical fibre 11 to a detector (12) suchas described in Wake D: "A 1550 nm Millimeter--wave Photodetector with aBandwidth Efficiency Product of 2.4 THz". (Journal of LightwaveTechnology, 1992, Vol 10 pages 908-912). The output from this detectorwas amplified by a Celeritek CSA946892 amplifier (13) and transmittedfrom a standard gain 20 dBi horn antenna as a microwave transmission inthe 28 GHz band.

While the embodiments described above have all included Mach-Zehnderinterferometers, those skilled in the art will appreciate that theconfiguration of the interferometer is not significant; any type ofinterferometer which exhibits non-linear transmission characteristicssuch as electro-absorption modulators may be used instead. All that isrequired is that it should exhibit an appropriate transmissioncharacteristic.

I claim:
 1. A method of generating an optical signal having a first RFcomponent including a first RF carrier frequency and an informationcomponent, the method comprising the steps of:i) generating a firstoptical signal having a second RF component including a second RFcarrier frequency different from said first RF carrier frequency; ii)generating a control signal having a third non-zero RF componentincluding a third RF carrier frequency different from said first RFcarrier frequency; said second or said third RF component including theinformation component; iii) applying the first optical signal to anoptical modulator; and iv) applying the control signal to the opticalmodulator to modulate the first optical signal so as to produce anoutput optical signal modulated by said first RF carrier frequency andsaid information component, said first carrier frequency being saidsecond carrier frequency up-converted by said third carrier frequency orby an integer multiple of said third carrier frequency, wherein anamplitude of said control signal is controlled to control a desiredharmonic output.
 2. A method according to claim 1, wherein the firstoptical signal includes the information component.
 3. A method accordingto claim 1, wherein the control signal includes the informationcomponent.
 4. An electrical or radio signal having a first RF carrierfrequency and an information component, generated by detecting anoptical signal modulated by an RF component comprising a first RFcarrier frequency and the information component, the optical signalbeing generated by the method of claim
 1. 5. A method according to claim1 wherein the control signal is an electrical signal.
 6. A methodaccording to claim 1 wherein the optical output of the modulator has anon-linear response to the control signal.
 7. A method according toclaim 6, wherein the amplitude of the control signal is selected suchthat the second RF carrier frequency is upconverted to the first RFcarrier frequency by a frequency which is an integer multiple of thethird RF frequency.
 8. A method according to claim 7 wherein the firstoptical signal is generated by controlling the bias voltage of a laser.9. A modulated optical signal having a first RF component including afirst RF carrier frequency and an information component, when generatedby the method of claim
 1. 10. A method of generating an encryptedoptical signal having a first RF component including a first RF carrierfrequency and an information component, the method comprising thefollowing steps:i) generating a first optical signal having a second RFcomponent including a second RF carrier frequency different from saidfirst RF carrier frequency; ii) generating a control signal having athird RF component including a third RF carrier frequency different fromsaid first RF carrier frequency; said second of said third RF componentincluding the information component, iii) applying the first opticalsignal to an optical modulator and iv) applying the control signal tothe optical modulator to modulate the first optical signal so as toproduce an output optical signal modulated by said first RF carrierfrequency and said information component, said first carrier frequencybeing said second carrier frequency up-converted by said third carrierfrequency or by an integer multiple of said third carrier frequency,wherein both the input optical signal and the control signal include aninformation component, one of which is a predetermined encryption code.11. A method of de-encrypting a signal according to claim 10 whereinsaid modulated output optical signal has a first RF component includinga first RF carrier frequency and an information component and furthercomprising applying to the signal a further modulation complementary tothat of the predetermined encryption code.
 12. An apparatus forgenerating an optical signal having a first RF component, which includesa first RF carrier frequency and an information component, saidapparatus comprising:i) an optical modulator having an optical input,and optical output, and a control input; ii) means for supplying to theoptical input a modulated optical signal having a second RF componentincluding a second RF carrier frequency different from said first RFcarrier frequency; iii) means for supplying to the control input acontrol signal having a third non-zero RF component including a third RFcarrier frequency different from said first RF carrier frequency; andiv) means for applying a modulation comprising said informationcomponent to said optical signal or said control input; the arrangementbeing such that there is produced at the optical output an opticalsignal modulated by said first RF carrier frequency and said informationcomponent, said first carrier frequency being said second carrierfrequency up-converted by said third carrier frequency or by an integermultiple of said third carrier frequency, wherein an amplitude of saidcontrol signal is controlled to control a desired harmonic output. 13.Apparatus according to any of claim 12, wherein the means for supplyingthe modulated optical signal comprises a laser and means for controllingthe bias voltage of the laser for generating the second RF component.14. Apparatus according to claim 12 wherein the control input is anelectrical input.
 15. Apparatus according to claim 14, wherein theoptical modulator is a Mach-Zehnder interferometer.
 16. Apparatusaccording to claim 15, wherein the means for supplying the controlsignal is arranged to generate a signal such that the second RFcomponent is upconverted by a frequency which is an integer multiple ofthe third RF component to generate the first RF component.
 17. Anencryption device for generating an encrypted optical signal having afirst RF component, which includes a first RF carrier frequency and aninformation component, said device comprisingi) an optical modulatorhaving an optical input, an optical output, and a control input; ii)means for supplying to the optical input a modulated optical signalhaving a second RF component including a second RF carrier frequencydifferent from said first RF carrier frequency; iii) means for supplyingto the control input a control signal having a third RF componentincluding a third RF carrier frequency different from said first RFcarrier frequency, iv) means for applying a modulation comprising saidinformation component to said optical signal or said control input; thearrangement being such that there is produced at the optical output anoptical signal modulated by said first RF carrier frequency and saidinformation component, said first carrier frequency being said secondcarrier frequency up-converted by said third carrier frequency or by aninteger multiple of said third carrier frequency; and means for applyingmodulation comprising information components to both the optical signaland the control signal, one of the information components being apredetermined encryption code.
 18. A method of generating an outputoptical signal having an RF modulation, said method comprising:applyingan input optical signal to an optical input of a modulator having anon-linear transfer function; and applying a control signal having acontrol RF frequency to control input of the modulator; the amplitude ofthe control signal being controlled to control a desired harmonic outputsuch that the output optical signal is modulated by an RF frequencywhich is an integer multiple of the control RF frequency.
 19. A methodaccording to claim 18, wherein the modulator is a Mach-Zehnderinterferometer.
 20. A method according to claim 18, wherein the inputoptical signal carries an initial RF modulation, the output opticalsignal comprising an RF modulation which is the initial RF modulationupconverted by an integer multiple of the control RF frequency.
 21. Amethod of generating an electrical or radio signal having a first RFcomponent including a first RF carrier frequency and an informationcomponent, the method comprising the following steps:i) generating afirst optical signal having a second RF component including a second,non-zero, RF carrier frequency different from said first RF carrierfrequency; ii) generating a control signal having a third RF componentincluding a third, non-zero, RF carrier frequency different from saidfirst RF carrier frequency; said second or third RF component includingthe information component; iii) applying the first optical signal to anoptical modulator; iv) applying the control signal to the opticalmodulator to modulate the first optical signal so as to produce anoutput optical signal modulated by said first RF carrier frequency andsaid information component, said first carrier frequency being saidsecond carrier frequency up-converted by said third carrier frequency orby an integer multiple of said third carrier frequency, wherein anamplitude of said control signal is controlled to control a desiredharmonic output; and v) extracting from said optical signal the first RFcomponent having said first carrier frequency and information component.22. An apparatus for generating an electrical or radio signal having afirst RF component, which includes a first RF carrier frequency and ininformation component, said apparatus comprising:i) an optical modulatorhaving an optical input, an optical output, and a control input; ii)means for supplying to the optical input a modulated optical signalhaving a second RF component including a second, non-zero, RF carrierfrequency different from said first RF carrier frequency; iii) means forsupplying to the control input a control signal having a third,non-zero, RF component including a third RF carrier frequency differentfrom said first RF frequency; iv) means for applying a modulationcomprising said information component to said optical signal or saidcontrol input; v) means for transmitting the optical signal to adetection means; and vi) the detection means being arranged to extractfrom the optical signal an RF component including said first carrierfrequency and information component; the arrangement being such that thedetection means extracts a signal modulated by said first RF carrierfrequency and said information component, said first carrier frequencybeing said second carrier frequency up-converted by said third carrierfrequency or by an integer multiple of said third carrier frequency,wherein an amplitude of said control signal is controlled to control adesired harmonic output.
 23. A method for generating an up-convertedmodulated RF carrier on an optical carrier signal, said methodcomprising the steps of:applying a first RF sub-carrier-modulatedoptical input at a first non-zero sub-carrier radio frequency to anoptical modulator; applying a second RF sub-carrier electrical input ata second non-zero sub-carrier radio frequency to said optical modulator;at least one of said first and second RF sub-carriers having beenmodulated to carry data therewith; and providing an optical carrieroutput signal from said optical modulator including an upconverteddata-modulated third RF carrier frequency higher than either said firstand second sub-carrier RF frequencies.
 24. A method as in claim 23wherein said optical modulator has a periodic transfer function and saidupconverted third RF frequency includes a component having a frequencyequal to said second RF frequency multiplied by an integer greater thantwo as determined by the amplitude of said electrical input. 25.Apparatus for generating an upconverted data-modulated RF carrier on anoptical carrier signal, said apparatus comprising:an optical signalsource providing a first RF sub-carrier-modulated optical signal at afirst non-zero sub-carrier radio frequency; an electrical signal sourceproviding a second RF sub-carrier electrical signal at a second non-zerosub-carrier radio frequency; an information signal modulator connectedto modulate at least one of said first and second RF sub-carriers withdata; an electro-optical signal modulator having an optical inputconnected to receive said first RF sub-carrier-modulated optical signal,having an electrical control input connected to receive said second RFsub-carrier electrical signal, and having an optical output providing anupconverted data-modulated RF carrier at a third RF carrier frequencyhigher than either of said first and second RF sub-carrier frequencies.26. Apparatus as in claim 25 wherein said modulator has a periodictransfer function and further comprising:means for controlling theamplitude of said second RF sub-carrier electrical signal to therebycause the upconverted RF frequency to include a component having afrequency equal to said second RF sub-carrier frequency multiplied by aninteger greater than two.